1. Field of the Invention
The present invention relates to rotary joints for fluids, and more specifically to rotary joints which allow solid-liquid mixture fluids, including slurry fluids (such as polishing solutions for polishing the surface of a silicon wafer by a chemical mechanical polishing technique, CMP for short) and corrosive fluids, to flow through the relatively rotating components and which can be connected to a monitoring apparatus for checking the polished surface state of the silicon wafer.
2. Description of the Prior Art
An apparatus for polishing the surface of silicon wafer by CMP to which this invention relates was recently developed. The apparatus, as shown in FIGS. 5 and 6, comprises: a rotary table 102 that rotates horizontally; a pad shaft support block 103 which moves back and forth and up and down; a polishing pad shaft 104 which, held by the shaft support block 103, is forced to rotate; a slurry fluid feeding and discharge passage 105 formed on the non-rotary side in the pad shaft support block 103; a polishing solution feeding and discharge mechanism 107 connected to the slurry fluid feeding and discharge passage 105 for feeding and discharging a polishing solution 106, for example, a KOH-containing silica slurry to which isopropyl alcohol is added; a slurry fluid feeding and discharge passage 108 on the rotary side which runs through the polishing pad shaft 104 and opens under a pad head 104a; and a rotary joint 111 which, installed between the pad shaft support block 103 and the polishing pad shaft 104, connects the two slurry fluid feeding and discharge passages 105 and 108 in a way that the two passages 105 and 108 communicate with each other and are relatively rotatable.
In that surface polishing apparatus, the silicon wafer 109 is polished in this manner. First, the silicon wafer 109 is held on the rotary table 102, surface 109a side up, and the polishing pad shaft 104 is moved down until the pad head 104a comes into contact with the wafer surface 109a. Then the polishing solution 106 is jetted into between the pad head 104a and the wafer 109 by means of positive pressure action (discharging operation of the polishing solution pump) of the feeding and discharge mechanism 107. The polishing pad shaft 104 is rotated and moved back and forth horizontally to polish the wafer surface 109a. After the polishing is over, the feeding and discharge mechanism 107 is switched over to negative pressure action (suction action of the polishing solution pump) to suck and discharge the residues of the polishing solution 106 into the slurry fluid feeding and discharge passages 105 and 108. That is, care is taken so that the residues of the polishing solution 106 in the slurry fluid feeding and discharge passages 105 and 108 may not drop on the polished surface of the wafer, and that is effected by switching the passages 105 and 108 from the positive pressure mode to the negative pressure or dry mode.
The rotary joint 111 mounted in that surface polishing apparatus is constructed as follows. A joint block mounted on the pad shaft support block 103 and a rotator assembly fixed on the polishing pad shaft 104 are connected in a manner which permits relative rotation of the two. Within the joint block is formed a first fluid passage section which is connected to the slurry fluid feeding and discharge passage 105 on the non-rotary side. On the rotary side, a second fluid passage section is formed in the rotator assembly and is connected to the slurry feeding and discharging passage 108. A space formed between the opening ends of the two fluid passage sections is sealed by seal units placed between the relatively rotating faces of the joint block and the rotator assembly. An example of such seal is a one in which relatively rotating parts of the joint block and the rotator assembly have sealing faces to be brought into contact with and pressed against each other, or an end face contact-type mechanical seal placed therebetween.
The rotary joint 111 of such a design presents many problems. That is, the polishing solution 106 is a slurry fluid containing abrasive grains. Those abrasive grains tend to intrude into and be deposited between the sealing faces (in the case of a mechanical seal, the opposing end faces of the two seal rings), making it difficult to maintain good sealing performance for a long period. A solid-containing slurry fluid, the polishing solution 106 wears out the seal faces fast, shortening the life of the seal. In the case of a mechanical seal, because the metallic parts such as a spring to thrust one seal ring against the other seal ring are exposed in the fluid passage, the solid matter in the polishing solution 106 comes into contact with the metallic parts. As a result, the abrasive grains in the polishing solution 106 impact against and remove microscopic protrusions on the surface, thus generating metallic particles or dust. The metallic particles are adsorbed to matter in the slurry fluid, thereby generating metallic ions. In the case of a corrosive fluid, the metallic particles could be corroded. If such metallic particles and dust or particles removed from the seal faces by wear are mixed with the polishing solution 106 and blasted from the pad head 104a, it will naturally have undesirable effects on the polishing of the wafer surface 109a. The entry and deposition of abrasive grains between the seal faces, the wearing of the seal faces, and the like occur noticeably when slurry feeding and discharging passages 105 and 108 are switched over from positive pressure mode to negative pressure or dry mode as mentioned above. Especially in dry mode, the seal contact faces could be heated and subjected to seizure because of frictional heat. As the intrusion and deposition of abrasive grains, wear of sealing faces, etc. affect the sealing performance, polishing solution 106 can leak through the seal faces, contaminating the wafer surface 109a, or get into the bearing placed between the joint block and the rotator assembly, hindering rotation of polishing pad shaft 104. Good polishing then becomes difficult to achieve.
In recent years, meanwhile, higher precision surface polishing has been demanded in accordance with a recent trend toward high integration. To raise the precision of polishing a wafer surface 109a, it is desirable to check and know the state of the wafer surface 109a during polishing and to control the polishing conditions including the polishing rate of the pad head 104a. To be specific, it is preferable to provide a pad head 104a with an appropriate polished surface detector 110 such as a monitor at the place indicated by the broken line in FIG. 6. The state of the wafer surface 109a is checked by detector 110 in real-time so as to control the polishing conditions properly on the basis of the detected surface state. To mount such a polished surface detector 110 in the rotating member pad head 104a, however, it is necessary to form a wiring route from the power source unit (including a display unit such as a monitor display apparatus to show the finding detected by the polished surface condition detector 110 and an operation panel and control panel to control polishing condition) to the polished surface detector 110 through the rotary joint 111 in such a way that the electric wire will not be twisted or damaged. In the surface polishing apparatus using the aforesaid rotary joint 111, it is impossible to form such a wiring route and to provide a pad head 104a with a polished surface detector 110.
Those problems with the rotary joint 111 are encountered not only in the aforesaid surface polishing apparatus but are common to rotary equipment in which a slurry fluid-like polishing solution or a corrosive fluid has to be blown between component parts relatively rotating at a rate higher than a certain level. Such being the case, it has been strongly desired that a solution to the problems should be found, but the fact is that no rotary joint for fluids has been developed which exhibits a stabilized sealing performance for a long time.